Acoustic horn



April 5 1927.

WITNESSES:

J. SLEPIAN ET AL ACOUSTIC HORN Filed Feb. 4. 1924 wmxmm' v |NVENTORSJose 1b Sleep/an 8r g l/nzon R. Han/7d ATTORNEY riod shall be materiallylessened.

Patented Apr. 5, 1927.v

UNITED STATES PATENT OFFICE.

ASSIGNORS T0 WESTINGHOUSE ELECTRIC .ANiD MAIN'UFACTURING COMPANY, A.

CORPORATION OF PENNSYLVANIA.

ncous'rro HORN.

Application filed February 4, 1924. Serial No. 690,405.

This invention relates to horns of the type used with sound-reproducinginstruments such as phonographs, loud-speaking telephones and the like.

It is an object of this invention to improve the volume of tone and thequality of the sound, delivered through such horn, from a vibratingdiaphragm. Y

The energy radiated from the horn in the form of sound-waves is thequantity which determines-how loud the sound seems to the listener. Thisdepends not only upon the shape and size of the horn, the size of thediaphragm and the volume of the space above the diaphragm, but also uponthe velocity of the movement of the diaphragm. The relation between theenergy radiated during a given short period and the average velocity ofthe diaphragm during said period, may be expressed as a ratio, which isreferred to herein as the intensification.

It is a further object of this invent-ion to produce a horn which shallgive an intensification that is substantially constant over a greatrange of pitch.

In all instruments including a horn and a diaphragm, a function of thehorn which has frequently been overlooked is to load the diaphragm bycompelling it to do work in moving the air within the horn and thusdamping the motion of the diaphragm. It -is a further object of thisinvention to so proportion this acoustic damping of the diaphragm to theother dampingfactors that the sound distorting effect ofthe resonance ofthe diaphragm at its natural pe- By making the acoustic damping large,we are enabled not only to diminish the radiated power at diaphragmresonance, but to increase it at frequencies other than resonance.

In this way, we produce a more uniform output throu hout arrange offrequencies without diminishing the average output.

It is a further object of this invention. to provide acoustic damping tosuch degree as to prevent the diaphragm from vibrating beyond theamplitude within which the restoring forces are linear functions of thedisplacement. If the diaphragm be permitted to vibrate more than this,the characteristic frequencies of the diaphragm itself begin toinfluence the output and distortion of the sound to be reproducedresults. This object of the invention is, therefore, to providesuflicient acoustic damping to prevent the amplitude of movement of thediaphragm from ever becoming great enough to introduce such distortionsas can result from the characteristics which greater movements of thediaphragm possess.

It is a further object of this invention to so correlate the size of theopening at the large end of the horn to the length of the (horn and thesize of the small end that suflicient radiation shall be obtainedwithout requiring a horn of impractical length.

We have discovered, by mathematical analysis and confirmed byexperiment, that the shape of the horn should be such that the area ofthe cross-section is an exponential function of the distance from thesmall end of the horn.

The rate of increase of the cross-section of a horn having anexponential form plays an important part, not heretofore-Linden.

stood, in determining alower limit to the range of pitch over which theintensification will be uniform. Said rate also fixes" the The size of iwill cease to be equal to the volume displaced by the movement of thediaphragm. When these two volumes begin to depart from equality, furtherdiminutions in the size of the throat does not result in acorresponding.;,increase in acoustic'damping. If the size of" the throatrelative to the volume above the diaphragm be too small, the veloeity ofair in the throat exceeds a certain critical value. Above this criticalvalue, turbulence occurs, with corresponding increase in frictionlosses. The lower limit of the size of the throat is fixed byconsideration, both of the necessity of equality between the two volumesexplained above and of the necessity for avoiding turbulence.

- the volume of the chamberbetween the dia- 1phragm and the throat, thanhave heretofore een usual. In the sound-reproducing instruments of theprior art, it has been usual to have a cylindrical space between thediaphragm and the cover which opens into the horn. A oone contains onlyone-third the volume resent in a cylinder of the same base and eight andwe reduce the volume of the space abovethe -diaphragm still more bymaking it approximately conical. The height of the cone in theinstrument we have devised is much less than the height of the space (ofwhatever shape) above the diaphra m in instruments used hitherto.

0t er features of our invention and details of the construction will bereadily understood from'the following description and the accompanyingdrawing in which Fig. 1 is a view in side elevation of aninstrumenteembodying this invention, and

Fig. 2 is a view on a larger scale partially V in section and partiallyin elevation of the operating instrument and a portion of the horn. V

A casing 1 contains mechanism for vibratinga diaphragm 2. This may bethe electromagnetic mechanism of a loud speaker, a phonograph or anyother means for actuating the diaphragm. A stem 3 is shown by .way ofexample to indicate means by which the mechanism within the case 1actuates the dia hragm 2. l

e diaphragm is held in place by a cap 4 I which is secured to the casing1 in any desired manner. The edges of the diaphragm are clamped betweenthe cap and the casing.

The clamping action is cushioned by means of soft rubber washers 5 and6. A recess is provided in the upper part of the cap in order toaccommodate the upper Washer. Preferably, the wall of this recessextends down over the inner surface of the washer, almost intoengagement with the diaphragm. From the-lower edge of this wall, the inside of the cap slopes upwardly and inwardly, forming a cone 7. Thedrawing shows the height of this cone as small as can be illustratedwith clearness. Actually, the height of the cone is relatively less.Preferably, it is only a few times the amplitude of the maximum movementof the d aphragm.

much smaller in this instrument than in instruments having horns of theform used is at the apex of the cone. This opening is much smaller thanthose in the horns used heretofore.

The horn 10-fits in the opening 8. The shape of this horn is given bythe equation:

AzA e it, we select-a value for B which will prevent the length of thehorn from being too inconvenient, when the areas of the ends are fixed,but this selection is based largely on considerations stated below. Theselection must be a compromise between the range of pitch over' whichthe horn will give uniform results and a convenient length.

We have found that for horns of exponential shape, B, the rate ofincrease of the cross-section, plays an important part in determiningthe lower limit of the range of pitch over which the intensification issufliciently constant to prevent noticeable distortion. This range isfrom very high pitches down to a pitch fixed by 2; I 4 B 2.5 X 10 wherem is 211' times the number of cycles per second for that pitch, and B iscomputed for a length measured in centimeters. The power radiated beginsto diminish before this point is reached and falls off rapidly as thepitch extends below this limit; but even so, the power radiated at lowpitch is greater than for horns of other shapes. For example, it reatlyexceeds that radiated by a conical liorn, having the same length andends of the same respective areas at all low pitches except those verynear the lower limit of audition.

The smaller the value of B, the lower the pitch at which efifectiveradiation begins to fall ofi that is, the better the quality of thereproduction. On the other hand, a smaller value of B requires a longerhorn for given sizes of the ends.

The size of the small end of the horn plays an important part indetermining the acoustic damping. Its area is. given by the foran A Jn 1B0 in which p is the density of air, a the velocity of sound in air, nthe permissible ratio of distortion, A, the diameter of the diaphragm,00 the period of the lowest frequency which the horn is expected toradiate, and B5, is the stiffness of the diaphragm. The stiffness of adiaphragm is the force tending to return it to normal position when ithas been displaced. I

The ideal instrument would radiate the same power at all pitches for thesame force on the diaphragm. Such an instrument has not. yet beendiscovered. With all known instruments there is more power radiated atcertain frequencies than at others for the same force on the diaphragm.The ratio between the power radiated at the frequency A practical hornwhich gives very good results has been constructed in which 13:.07 forc. m. units'of length, which corresponds to approximately twenty percent increase in area of cross-section per inch of length. This gave afalling-soft point at w:1250, which corresponds to a pitch somewhatabove middle 0. The diameter of the small end is 0.508 cm. or 0.20inches. The length is 132 cm. or about 4 feet and the diameter at thelarge end 50.8 cm. or about inches.

If this volume be too large in proportion to the area of the small endof the horn, the motion of the diaphragm, instead of being completelytransformed into velocity of the air in the throat, may be partlyexpended in compressing the air within said volume.

To obtain the desired uniformity of response over a desired range, thechamber volume must be not greater than that given by the formula inwhich 1, corresponds to the pitch at which the diaphragm is resonant, mcorrevolume of thechamber above the diaphragm must theoretically be madesmaller than the physical diff culties of manufacture permit. w may,however, be as large as 40,000 without the difficulties due to thiscircumstance appearing.

The area of the large end-of the horn must be fixed by a compromisebetween the physical difficulties resulting from excessive length andthe acoustic advantages which result from having this end very large. Ifthe diameter of the final opening exceeds the wave length of the lowestpitch for which the horn is expected to correctly intensify, thereflections'which take place at the lar e end will be negligible; It isfound that this result is obtained when the radius of the large end isThis is the size which the .The lower limit of pitch for which thereflections are negligible is, with such a horn,

' the same as the pitch at which the intensification begins to fall offrapidly.

A horn of the dimensions stated above was used with a diaphragm havingan area of 15.5 sq. cm. and a stiffness of 20 X 10 dynes per cm. over arange from somewhat l 1 to: (01 L02 below middle 0 to l-line 0 (0). Theconical chamber above the diaphragm had a height of .05 cm. l/Vith aconstant current input, the greatest intensity throughout thepitch-range, was not more than ten times the smallest intensity. Such ahorn, used with a loudspeaking telephone, gave a reproduction of musicwhich was not onlypleasing, but very close to the original. It'gave areproduction of voice which was not only intelligible, but natural.

When the diaphragm. is actuated by a phonograph record, through a needleand lever system designed for use with an ordinary horn, greater powerwill be delivered from the record to the needle and greater volume ofsound emitted by the horn. If desired, a different mechanical ratio maybe employed in the needle-lever system, so that the displacement of thediaphragm is diminished. This would enable heavier memto use, with theordinary lever system, records prepared especially for use with such ahorn. The undulations or sinuosities in such records would be of smalleramplitude than heretofore, which-would be an advantage in themanufacture of such records. In all. of these ways of applying the hornto a phonograph, the advantages obtained with the ater acoustic dampingcauses greater free om from tendency to radiate disproportionately largepower at those pitches which correspond to the natural period of freevibration of the moving parts.

Although we have described and illustrated but one particular embodimentof our invention, it is understood. that changes within the spirit ofthe invention can be made by those skilled in the art. We, therefore, donot purpose limiting the invention except as necessitated by the priorart or indicated in the claims.

We claim as our invention:

1. An acoustic horn, having an exponential variation of cross-sectionwith the length and an internal diameter at the small end less than athird of an inch.

2. In combination, a diaphragm, a horn, an enclosure housing saiddiaphragm and opening into said horn, said opening being as small,relative to the volume of said enclosure, as may be without causingthevolumetric flow of air into said opening to be appreciably less thanthe volumetric rate of displacement of the diaphragm.

3. In combination, a diaphragm, means for vibrating said diaphragm, andacoustic means for so damping said vibration that the displacement ofthe diaphragm does not exceed an amount at which the restorative forcesthereof cease to have a linear relation walls making a maximum angle offortyfive degrees with the axis, and a throat, the minimum diameter ofwhich is so small that the acoustic damping is greater than the sum ofall other damping.

5. In combination, a diaphragm, a born, a chamber closed at one side bysaid diaphragm and opening into the horn, and means for vibrating saiddiaphragm, said chamber closely conforming to one extreme position ofsaid diaphragm and the area of said opening being so small that theacoustic damping of the diaphragm prevents excursion thereof beyond thepoint of linear coefiicient of stiffness.

6. In a sound-reproducing instrument, a horn, a diaphragm, an enclosureproviding a chamber between said diaphragm and horn, said chamberopening into the throat of said horn, the area of said opening beinglarge enough relative to the area of said diaphragm and volume of saidchamber, to ensure the volumetric flow of air into the opening shall beequal to the volumetric rate of displacement of the diaphragm, and smallenough to damp the motion of said diaphragm more than the sum of allother damping. I

7. An acoustic horn, having an exponential variation of cross-sectionwith-the length and an internal diameter at the small end less than twoper cent of the internal diameter at the large end.

8. An acoustic horn, the area of the crossscction of which is anexponential function of the distance from the small end, the diameter ofthe small end being less than onehalf of one per cent of the length ofthe horn.

9. An acoustic horn, having an exponential variation of cross-sectionwith the length, the rate of increase of cross-section per unit lengthbeing sufliciently small to bring the pitch limit of accuratereproduction within an octave of middle 0.

10. An acoustic horn, the area of whose cross-section is an exponentialfunction of the distance from the small end, the rate of increase of thecross-section being less than twenty-five per cent per inch and thediameter oi the opening at thesmall end being less than two per cent ofthe diameter of the opening at the large end.

11. In a sound-reproducing instrument, a diaphragm, a conical housingcovering said diaphragm, the altitude of the cone being less than oneper cent of its base-diameter.

12. In a sound-reproducing instrument, a diaphragm, a horn, a conicalhousing covering said diaphragm and opening into said horn, the altitudeof the cone being less than fifty times the maximum amplitude of themotion of the diaphragm and the said opening being so small that thediaphragm does not move beyond the point at which the. restorativeforces thereof become a non-linear function of the displacement.

13. In a sound-reproducing diaphragm, a conical housing covering saiddiaphragm and having an opening. at its apex, the altitude of the conebeing less than fifty times the maximum amplitude of the motion of thediaphragm.

14. In a sound-reproducing instrument, a diaphragm, a horn,a housingover said diaphragm opening into said horn, the height of said housingbeing less than fifty times the maximum amplitude of the motion of thediaphragm when loaded bysaid horn.

15. In a sound reproducing instrument, a chamber having an opening, andmeans cooperatingwith said chamber to produce an air movement in saidopening corresponding to the sound to be reproduced, the area of saidopening being such that the pressure within the chamber is maintainedsubstantially constant.

16. In combination, a diaphra m, a horn, an enclosure housing said diapragm and opening into said horn, said opening being instrument, a

tion, and the cross sectional area of the small end being such that theratio of the damping of the diaphragm by .said horn to the mass of thediaphragm is greater than 10,000 c. g. s. units. 7

18. In an acoustic instrument, a diaphragm chamber, a diaphragi'ntherein, a

born, the cross section of which is an exponential function of thedistance from the small end, the small end of said horn cominunicatingwith said diaphragm chamber, whereby the acoustic damping upon saiddiaphragm will be a function of the area of said small end of thehorn,.said area being so small that the acoustic damping per unit massof the diaphragm is sufficient to prevent the ratio of sound energyradiated at the natural resonance period of the diaphragm to the soundenergy radiated at the owest frequency at which the horn can effectivelyradiate to be less than a normal car can distinguish when listening tomusic.

15). In an acoustic instrument, a diahragm chamber, a diaphragm therein,a 10m, the cross section'of which is an exponential function of thedistance from the small end, the small end of said horn communicatinwith said diaphragm chamber, whereby t 1e acoustic damping upon saiddiaphragm Will be a function of the area of said small end of the horn,said area being so small that the acoustic damping is suflicient toprevent perceptible distortion over the range of frequency for which thehorn is inten ed. i

20. In an acoustic instrument a diaphragm chamber, a diaphragm therein,a horn, the cross section of which is an exponential function of thedistance from the small end, the small end of said horn communicatingwith said diaphragm chamber, whereby the acoustic damping upon saiddiaphragm will be a function of the area of said small end of the horn,aid area being so small that the acoustic damping is sufiicient to causethe radiated sound to be substantially independent of the frequenc fromthe highest frequencies for which'the instrument is intended to thecut-off frequency determined by the rate of increase of said horn.

21. In an acoustic instrument a diaphragnrchamber, a diaphragm therein,a horn, the cross section of which is an exponential function of thedistance from the small end, the small end of said horn communicatingwith said diaphragm chamber, whereby the acoustic damping upon saiddiaphragm will be a function of the area of said small end of the horn,said area being so small that the acoustig damping per unit gt: wherew,- isthe natural resonance frequency of the diaphragm and w, is thelowest frequency which the instrument is expected to radiate.

mass is greater than 22. In an acoustic instrument, a horn having across section which is an exponential 7 function of the distance fromthe small end, the rate of increase of said cross section being lessthan twent -five percent per inch and the area of the arge end being atleast equal to that of a eirele having for its radius, where B is theconstant in-the ex-i uary,1924.

1 JOSEPH SLEPIAN.

CLINTON R. HANNA.

